The principal drawback with air-filled vehicle tires, such as tires used on automobiles, trucks, tires, aircraft, fork-lift, trucks, tractors, etc. is that when the tire is punctured by a sharp object, it promptly loses air and goes flat. This necessitates having to change the tire. If the flat occurs late at night on an infrequently travelled road, the driver may experience difficulty in obtaining assistance to change the tire and replace it with a spare. If the flat occurs as a fork-lift truck is carrying a heavy load, the consequences can be a disaster.
The art has long sought to make vehicle tires with a solid filler in lieu of air so as to avoid having the tires go flat due to loss of air when punctured. The use of elastomeric fillings, including polyurethane fillings for tires wherein the filling is used in place of the air under pressure is known. Such tires are usually placed on heavy duty vehicles used in rugged terrains, including mining vehicles, construction equipment, land-clearing equipment, farm equipment, scrap yard vehicles and vehicles for off-road applications, etc. where the vehicles are constantly driven over rocks and debris.
U.S. Pat. No. 3,022,810 discloses the formation of a rubber-like closed-cell resilient polyurethane foam of substantially uniform density within a tire casing. U.S. Pat. No. 3,112,785 also discloses in situ forming of a polyurethane foam in a tire casing, which foam does not adhere to the inner wall of the casing and the adding of a liquid antifreeze solution into the tire to be dispersed into the cellular structure. U.S. Pat. No. 3,381,735 forms a synthetic rubber filler within the tire and then vulcanizes the filler. U.S. patent No. 3,605,848 describes a tire filled with a microcellular polyurethane core wherein water is utilized as the blowing agent and chain extender. Upon reaction of the water with the isocyanate, carbon dioxide is formed which results in the foaming of the polyurethane reaction mixture.
As a tire rotates on a vehicle wheel, it is cyclically compressed at a rate dependent upon the vehicle speed. Due to internal friction, or hysteresis, in the filler material, heat builds up in the material. Noncyclical distortions due to uneven terrain and turning of the vehicle also cause heat buildup in like manner. The ultimate temperature reached in the tire depends on the heat generated by hysteresis and on the thermal conductivity of the filler which causes heat to flow out of the filler, as well as on ambient temperatures and tire size. Degradation of the filler material under elevated temperature conditions and mechanical strain cause the breaking of the chemical bonds in the material, thereby weakening it and allowing the material when stressed to exhibit different elastic behavior. The major drawback with tires filled with polyurethane foam has been that they are readily flexed, thus causing excessive heat build-up within the tire as it is driven, causing filler breakdown, casing damage, etc. due to poor heat transfer. This high temperature degrades the rubber tire sheath within a short period of time.
U.S. Pat. No. 3,866,651 which was reissued as RE. 29,890, discloses the use of a void-free elastomeric polyurethane in a tire casing which overcomes some of the problems of polyurethane foam fillers described above. The patentee produces his elastomeric polyurethane material in situ within the tire casing in the essential absence of foam producing materials, i.e. blowing agents, in the reaction zone. In order to avoid formation of any pores in the polyurethane, stringent moisture-free conditions are required. The resulting filler imparts less deflection to the tire than does a foam filler, such that improved heat build-up characteristics are obtained. However, the drawback in using this essentially void-free polyurethane elastomeric material is the resultant very high cost of filling a tire. Attempts to reduce such costs by diluting the filler material with extender oil results in a sharp decrease in the hardness of the filler.
U.S. Pat. No. 4,416,844 discloses a method for making an essentially void-free polyurethane tire fill composition having extender oil as part of the composition while maintaining a Durometer hardness of at least 20 on the A scale for such composition. The patentee adds water as a reactant to produce carbon dioxide in the reaction zone, e.g. in the tire, and cures the elastomer under conditions whereby the carbon dioxide is dissolved in the elastomer. A pressure of at least 25 psi is required to prevent bubble formation in the tire. As a result of the water reaction, a polyurea-containing polyurethane elastomer is obtained having good hardness characteristics. The patentee states that while a polyurea-containing elastomer generally has less oil compatability on a weight basis than an all-urethane containing elastomer, because of its very high Durometer hardness, such polyurea-containing elastomer has higher oil compatability for a particular level of Durometer hardness. Whereas urethane systems can be oil-extended at a useful hardness level, the level of oil extension is limited by large decreases in hardness. However, elastomer prepared in accordance with the invention of U.S. Pat. No. 4,416,844 can be extended by as much as fifty weight percent with oil and still yield a Durometer hardness of 30 and, for lower hardness levels, up to sixty weight percent oil can be added without bleeding from the elastomer at room temperature. In each of the examples in U.S. Pat. No. 4,416,844 an isocyanate-polyol precursor was reacted with a polyol to form the polyurethane elastomer tire filling. In fact, all of the disclosures in the aforementioned patents use either a prepolymer in the isocyanate component or precursor or allude to the mixing of all of the reactants together and subsequently injecting the reaction mixture into the tire casing and curing it therein. All of the reaction processes in the foregoing patents also include the use of a catalyst for the reaction.